1. Field of the Invention
The present invention relates to an electric heating system for a motor vehicle.
2. Description of the Related Art
A heating system of that kind has been known from DE 199 02 050 A1 as an auxiliary heating system for a motor vehicle that is driven by a combustion engine. As a rule, motor vehicle heating systems utilize the waste heat produced by the combustion engine. However, the continuous progress made in increasing the efficiency of combustion engines has resulted in reducing the amount of waste heat available for the heating system. This is true especially for those direct-injection diesel engines which are designed especially for fuel economy. Due to their small yield of waste heat, these engines do not, especially in winter times, warm up the vehicle as quickly as would be desirable for defrosting the windscreen, to prevent fogging of the windscreen and to heat up the passenger compartment to a comfortable temperature. It is, therefore, known from DE 199 02 050 A1 to provide an electric auxiliary heating system with a power of, typically, between 250 Watts and 2,000 Watts. The know auxiliary heating system uses for this purpose a plurality of electric heating rods which are combined with the water-carrying main heating system of the vehicle to form a joint heat exchanger that transmits its heat to an air flow directed into the passenger compartment. However, heat exchangers and electric auxiliary heating systems are frequently implemented also as two separate units.
The electric heating rods are supplied with current of an intensity of, typically, 30 A and their output is controlled by a control circuit which controls the power of the auxiliary heating system with the aid of one or more power semiconductors. In their operative condition, the power semiconductors have a resistance which cannot be ignored, and accordingly produce heat, due to energy losses, in the order of 10 Watts each, for example. The heat loss encountered in the power semiconductors must be eliminated if damage to the control circuit and to any plastic components in its neighborhood is to be prevented. The control circuit, together with its power semiconductors and heat sinks, is arranged for this purpose on a circuit board, which is combined with the heat exchanger to a single unit which is positioned in the air current that flows through the heat exchanger. It is a disadvantage of that arrangement that being positioned in the air current, the control circuit is not protected from environmental influences, and electric safety is not guaranteed, and this the more as the air current may exhibit a high degree of humidity that may deposit on the control circuit temporarily.